Moving on now to fresh water animals. Once the ancestors of the
modern freshwater animals had made the transition to the freshwater environment,
presumably by way of the estuaries, there was no longer any great advantage
to their descendants in continuing to maintain body fluids as concentrated
as seawater, as long as they remained in their new environment. Such excessively
hypertonic internal conditions simply aggravated the problems of obtaining
enough salt and bailing out excess water. Thus it is understandable that
natural selection should have favored a reduction of the osmotic concentration
of the body fluids within the bounds possible for the continuance of the
life of the tissues, and that modern freshwater animals, both invertebrates
and vertebrate, have osmotic concentrations decidedly lower than seawater.
It seems incompatible with cellular existence, however, for the body fluids
to be as dilute as freshwater, for no organisms are actually isotonic with
their freshwater medium; the body fluids of freshwater animals are typically
hypotonic relative to seawater, but hypertonic relative to freshwater.
Now, if freshwater animals are hypertonic relative to the surrounding
environmental medium, there will be a strong tendency for water to move
into the organism and for salts to be lost from the organism to the surrounding
water. At first glance, the obvious evolutionary solution to this problem
might seem to be the development of completely impermeable membranes covering
the entire body, but further thought shows that this solution would have
been impracticable, since a truly aquatic organism must maintain some permeable
membranes exposed to the water for gas exchange. Because mammals that live
in the water breath air and hence need never expose permeable respiratory
membranes to the water, they can maintain an impermeable barrier between
their body fluids and the water in which they live. But fully aquatic freshwater
animals cannot use the "method of evasion" exclusively. They
must also be able to carry out active osmoregulation, which usually involves
excretory organs that can pump out the water as fast as it floods
in--- preferably through the production of urine more dilute than the body
fluids--- and/or special secretory cells somewhere on the body that can
absorb salts from the environment and release them into the blood. Both
corrective measures--production of dilute urine and absorption of salts
(minerals/metals) --entail movement of material against concentration gradients
and therefore necessitate expenditure of energy.
An examination of the water and salt regulation typical of modern
freshwater bony fishes will provide a good example of the before mentioned
processes. The bony fishes are the class Osteichthyes, most of the fish
familiar to you. They have backbones of vertebra. The blood and tissue
fluids of the fish are more concentrated than the environmental water.
The method of evasion is used to the extent that much of the body is covered
by relatively impermeable skin and scales, and that the fish almost never
drink. There is, however, a constant osmotic intake of water across the
membranes of the gills and of the mouth, and a constant loss of salts across
the same membranes. The method of correction is used two ways: The excess
water is eliminated in the form of very dilute and copious urine produced
by the kidneys, and salts are actively absorbed by specialized cells in
the gills.
Curiously enough, marine vertebrates, bony fishes living in the
seawater have the reverse problem: They live in water, yet they steadily
lose water to their environment and are in constant danger of dehydration.
The explanation is that the ancestors of the bony fishes (vertebra fishes)
apparently lived in freshwater, not in the sea, and that when some of
their descendants moved to the marine environment they retained their dilute
body fluids. Vertebra were a development of estuary animals, and moved
back to the sea and on to the land. Thus marine bony fishes are hypotonic
relative to the surrounding water, and they have the problem of excessive
water loss and excessive salt intake. Besides benefiting from the evasive
adaptation of relatively impermeable skin and scales, they use two corrective
measures: They drink almost continuously to replace the water they are
constantly losing, and, by means of specialized cells in the gills, they
actively excrete the salts they unavoidably take in with the water. Most
of the nitrogenous wastes are excreted as ammonia through the gills; hence
only a small quantity of urine is produced by the kidneys, and little water
need be lost in this manner. Apparently fish kidneys have not evolved the
capacity to produce concentrated urine, and they are consequently
of no help in salt elimination.
The marine elasmobranch fishes (sharks and their relatives) probably
also evolved from freshwater ancestors, but they solved the osmotic problem
in a very different way. Their blood contains about the same concentrations
of salts as the blood of marine bony fishes, but their blood also contains
high concentrations of urea, to which they are more tolerant than most
vertebrates. By conserving the urea instead of excreting it, the marine
elasmobranchs maintain a total osmotic concentration in their blood slightly
greater than that of the seawater. They therefore have no problem
of water loss. Excess salt is excreted by special glandular cells in the
rectum.
---to be continued---
Bless you, Bob Lee
--
oozing on the muggy shore of the gulf coast
[email protected]
--
The silver-list is a moderated forum for discussion of colloidal silver.
To join or quit silver-list or silver-digest send an e-mail message to:
[email protected] -or- [email protected]
with the word subscribe or unsubscribe in the SUBJECT line.
To post, address your message to: [email protected]
Silver-list archive: http://escribe.com/health/thesilverlist/index.html
List maintainer: Mike Devour
